In recent times, the development of new coating materials in the automotive, plastics, wood products, and industrial sectors has been directed toward two principal objectives: reducing the solvent emissions, and avoiding waste.
Accordingly, in the development of new coating materials, particular preference is given to modern coating compositions, such as high-solids, aqueous systems, and powder coatings.
The front runners in these trends in development are usually motor-vehicle manufacturers and the associated refinish trade. The four-coat automotive OEM finish employed nowadays consists of an aqueous cataphoretic deposition coating to provide protection against corrosion, an aqueous or solvent-containing filler for protection against stone chipping, an aqueous metallic basecoat for coloring and/or for the metallic effect, and a solvent-containing--usually high-solids, aqueous or powder--clearcoat which gives the coating the necessary gloss and resistance to water, chemicals and the effects of weathering.
It is the filler, basecoat and clearcoat coats in particular which are currently the subject of intensive research efforts, as described for example in documents DE-A 43 24 801, DE-A 41 22 265, DE-A 41 22 266, DE-A 43 36 206, DE-A 43 42 384, DE-A 43 26 656, DE-A 44 15 319, DE-A 43 11 128, EP-B 0 056 971, EP-A 0 509 392, and EP-A 0 509 393.
As regards the topmost coat, the clearcoat, there is competition between all three modern coating compositions, and in comparison each of these coating systems displays strengths and weaknesses both in application and in use. This final coat usually consists of acrylate copolymers, since these are very weatherproof. For high-solids binders these copolymers are of low molecular weight formulation; for aqueous binders they are usually neutralized and dispersed in water or, for powder binders, produced in pulverulent form. These acrylic resins are acrylate copolymers with or without styrene, which, depending on the crosslinking reaction (curing in each case includes melamine crosslinking, isocyanate cross-linking (blocked or unblocked), carboxyl cross-linking, epoxy cross-linking, etc., or any desired combinations thereof), contain appropriate functional groups.
Styrene as an aromatic vinyl monomer possesses the function of "hardening" (increasing the glass transition temperature) and of making the products less expensive. A disadvantage, however, is the reduction in weathering stability as the content of styrene increases. Efforts are therefore directed at limiting the content of styrene and employing other "hard and weather-resistant" monomers which lead to as a high as possible a glass transition temperature in the resin. What are desired, therefore, are products comprising appropriate monomers which possess as small a free volume as possible.
Monomers which are suitable are specific aliphatic acrylic esters which are sterically hindered, i.e., which possess either a branched or a (poly)cyclic structure in the side chain. These monomers have a "hardening" effect, since their homopolymers have glass transition temperatures of more than 45.degree. C, measured at a sufficiently high molar mass of the homopolymer at which there is no longer dependency of the temperature of the glass transition stage on the molar mass.
An acrylic monomer having a particularly high glass transition temperature is isobornyl acrylate or methacrylate. Conventionally, it can be prepared over two steps by transesterifying methyl methacrylate or methyl acrylate with isoborned, or by esterifying the alcohol with methacrylic acid/acrylic acid or chlorinated derivatives thereof. The first step in this case is the preparation of the isoborned by the acid-catalyzed reaction of water with camphene.
More recent methods for the preparation of these monomers in one step start directly from the precursor camphene and from methacrylic or acrylic acid, which are reacted on an acid ion exchanger which, in suspension, fills the reactor uniformly. (See U.S. Ser. No. 462.701, filed Jun. 5, 1995, hereby incorporated by reference.).
By setting the reaction parameters, this method leads to intensified rearrangement reactions on the camphene skeleton (Wagner-Meerwein rearrangements), which are able to produce relatively large quantities of structural isomers of isobornyl (meth) acrylate, such as pseudobornyl (meth)acrylate and isofenchyl (meth)acrylate.
The proportions by mass of the individual structural isomers, acrylates or methacrylates, according to the new 1-step process as compared to the 2-step process are:
______________________________________ Process (Meth)acrylate 1-step (new) 2-step ______________________________________ Isobornyl 50-92% 92-99.9% Isofenchyl .gtoreq.3.5% &lt;3.5% Pseudobornyl .gtoreq.1% &lt;1% ______________________________________